Raspberry Pi 3s can be configured to boot off external USB mass storage devices. USB / SATA adapters are cheap and readily available, as are standalone SATA drive cloning devices. SATA SSDs are cheaper than USB SSDs and offer excellent random seek performance vs magnetic - which is very useful for querying UTXOs in a memory-constrained device. They offer higher throughput, larger capacities, cheaper price points and wear leveling vs microSDs. Dual bay SATA cloning devices greatly expedite the process of backing up to cheaper magnetic drives, and their linear technique does not require fast seek performance in the backup drive.
This solves the IO bottleneck sufficiently that a Pi 3 becomes processor-constrained during full node initialization. A passive heat sync does not suffice if you want to run this setup at full throttle, you will need a fan during that initial sync. Once you are caught up of course, you can slow down and perhaps switch back to a cheaper magnetic drive.